Clemens in U.S. Pat. No. 3,842,194 described a capacitance video disc system in which the disc, which was made of a thermoplastic material that was a vinyl resin, had a conductive metal coating to provide the conductivity required for capacitative pickup and a dielectric material thereover. An electrode on the playback stylus completed the capacitor.
Improvements in this system in which the disc is made from a conductive plastic material have been disclosed. Khanna in U.S. Pat. No. 3,960,790 taught a method of making disc record molding compositions by compounding a vinyl chloride-vinyl acetate copolymer and a vinyl chloride homopolymer with carbon black, a stabilizer, a plasticizer, an antistatic agent, and a mold release agent in a series of blending steps that included successive increases in temperature. In U.S. Pat. No. 4,129,536, Martin et al. disclosed injection molding compositions that comprised a vinyl chloride-propylene copolymer or polyvinyl chloride, a solid mercaptotin stabilizer, an acrylic resin processing aid, and an ester wax lubricant and the use of these compositions in the production of video discs. Molding compositions suitable for use in the compression molding of video discs that contained a vinyl chloride-vinyl acetate copolymer, a vinyl chloride-maleate ester copolymer, a vinyl chloride-propylene copolymer, conductive carbon black particles, polymeric plasticizers and processing aids, two or more metal stabilizers, and three or more lubricants were disclosed by Khanna in U.S. Pat. No. 4,151,132. While the molding compositions of Khanna are readily processable and moldable to form video discs having excellent playback characteristics, they are unsatisfactory because they have a relatively low heat distortion temperature, which results in warpage and shrinkage when the discs are stored at temperatures above 37.degree. C. Molding compositions said to have improved dimensional stability at 54.degree. C., homogeneity, and surface characteristics were disclosed by Martin et al. in U.S. Pat. No. 4,228,050. These compositions comprised polyvinyl chloride or a vinyl chloride--propylene copolymer having a heat distortion temperature of 60.degree. C. or higher, conductive carbon black particles, stabilizers, lubricants, plasticizers, and processing aids, with the proviso that not more than about 5% by weight of liquid additives that are compatible with the vinyl chloride resin are present.
While all of the conductive molding compositions of the prior art can be used to form video discs, none of them meets all of the requirements that have been established for compositions that are to be used in this application, that is, good processability and thermal stability during compound processing and molding, good conductivity, good replication of submicron size surface relief patterns, and dimensional stability on storage under various environmental conditions.